Drainage or katabatic flows are common mesoscale circulations established as a result of differential radiative cooling of near-surface air masses in sloping terrain. The initial irruption of these flows, with sudden shifts in wind speed and direction, may result in vertical displacements of air parcels from their equilibrium position, which prove to be a common source of internal gravity waves. This paper illustrates this mechanism and describes the main features of the oscillations following the study of observational data gathered throughout one night during the Stable Atmospheric Boundary Layer Experiment in Spain 2006 (SABLES2006) field campaign.

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This research has been funded by the Spanish Ministry of Science and Innovation (Projects CGL2004-03109, CGL 2006-12474- C03-03, and CGL2009-12797-C03-03). The GR58/08 program (supported by BSCH and UCM) has also partially financed this work through the Research Group ‘‘Micrometeorology and Climate Variability’’ (910437).